The present invention relates to catalytic converter substrates and more particularly relates to a strengthened catalytic converter substrate and a method for preparing the substrate.
A variety of ceramic and metal catalytic converter substrates are known. Commercially available ceramic catalytic converter substrates have thin walls (typically about 0.006 inch to about 0.008 inch) and shaped cells, such as round, square, or triangular shaped cells. The substrate is formed by extruding a green ceramic mixture through an extrusion die. The extruded plasticized material is then dried and fired to provide a hard, solid catalytic converter substrate. Any ceramic material having suitable thermal, shock resistance, and melting temperature characteristics, such as cordierite, can be used. A washcoat is applied to the fired substrate, and catalyst solutions are impregnated into the washcoat. To prepare the catalytic converter, the catalyst-coated substrate is clamped into a suitable catalytic converter housing with a mat (which may have an expanding component) or other resilient retention material compressed between the substrate and the housing.
The catalyst washcoat is of a known type that, upon reaching a light-off temperature (i.e., effective catalytic operating temperature) stimulates reactions between constituents in exhaust gas flowing through the substrate cells to reduce the presence of undesirable species within the exhaust gas. In vehicle applications, hot engine exhaust gas flowing through the substrate raises the catalyst temperature to achieve light-off temperature. In the time period before any part of the substrate reaches light-off temperature, the catalytic converter is not operational to stimulate the reduction of undesirable species in the exhaust gas and those undesirable species escape from the tailpipe into the environment. This occurs, for example, when an engine is started from a xe2x80x9ccold-start,xe2x80x9d i.e., when the engine has not been running in a while and the catalyst temperature at engine start-up is substantially that of the surrounding environment.
Accelerated catalyst heating is desirable and is particularly important for meeting increasingly stringent state and federal government vehicle emissions standards, such as the SULEV (Super Ultra Low Emission Vehicle) emissions standards proposed for 2004 introduction in California. For example, in normal, light load, summer operating conditions represented by the U.S. FTP (United States Federal Test Procedure), a vehicle developed and certified to the SULEV level emits extraordinarily low emissions comparable to an electric vehicle (including power plant emissions) and reactive hydrocarbon emissions modestly higher than an electric vehicle.
The time it takes for the substrate to reach light-off temperature depends, in part, upon the thermal mass of the substrate. A substrate with a lower thermal mass is heated to operational temperature quicker than a substrate with a higher thermal mass. One method of reducing the thermal mass of a substrate is to reduce the thickness of the substrate walls. However, ultra-thin walled substrates have very low strength. When the cell walls are too thin, the substrate will not be robust to assembly processes. Further, when the cell walls are too thin, the substrate will lack sufficient structural strength to survive in its operating environment. For example, catalytic converters are exposed to continual mechanical stresses, namely vibrations (particularly when used in automobiles), as well as thermal expansion and contraction (from constantly changing operating conditions, especially stop and go operation).
There is a need in the art for an improved catalytic converter substrate and method for preparing the same. Particularly, there is a need in the art for an improved catalytic converter substrate and method providing low thermal mass for fast light-off in combination with sufficient structural integrity to survive fabrication and assembly processes and to remain intact in harsh automotive environments over its intended service life.
The present invention provides a strengthened catalytic converter substrate and a method for preparing the substrate using controlled, selective washcoat application to maximize the structural integrity of the thin-walled substrate. The catalytic converter substrate comprises a substrate having cells defined by thin perimeter walls and thin interior walls, and a catalyst washcoat selectively disposed on the substrate so as to maximize substrate strength in those areas requiring the greatest amount of structural integrity.
The present invention employing selective washcoat application provides the advantage of lower substrate cost as compared to substrates that are strengthened with thicker walls or thicker applied skins. The present invention further provides the advantage of a more robust product design and reduces the amount of fallout due to cracked catalyst.
The present invention provides the further advantage of reduced processing costs. For example, since there is a high degree of die wear when extruding material such as cordierite, it is desirable to reuse dies for smaller cross-section parts as the outer cells wear out. This is not readily accomplished when wall thickness is different in outer cells versus inner cells. The present invention reduces the need for new extrusion dies by providing added strength to the catalyst where it is needed (e.g., in perimeter cells) without the need for differential wall thickness.
The present method includes using known coating methods including use of a mask for selectively disposing the washcoat. This provides a further advantage in that tooling costs for washcoat masking are significantly less than the costs of providing substrate walls having different wall thicknesses.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.